SUMMARY Beta oscillations (12-30Hz) in local field potentials are prevalent in the motor system, yet their functional role within the context of planning a movement is still debated. In this study, a human participant implanted with a multi-electrode array in the hand area of primary motor cortex (MI) was instructed to plan a movement using either the second or fourth of five sequentially presented instruction cues. The beta amplitude increased from the start of the trial until the informative (second or fourth) cue, and was diminished afterwards. Moreover, the beta amplitude peaked just prior to each instruction cue and the delta frequency (0.5-1.5Hz) entrained to the interval between the cues - but only until the informative cue. This result suggests that the beta amplitude and delta phase in MI reflect the subject’s engagement with the rhythmically-presented cues and work together to enhance sensitivity to predictable and task-relevant visual cues.

Interactions among groups of neurons in primary motor cortex (MI) may convey information about motor behavior. We investigated the information carried by interactions in MI of macaque monkeys using a novel multielectrode array to record simultaneously from 12-16 neurons during an arm-reaching task. Pairs of simultaneously recorded cells revealed significant correlations in their trial-to-trial firing rate variation when estimated over broad (600 msec) time intervals. This covariation was only weakly related to the preferred directions of the individual MI neurons estimated from the firing rate and did not vary significantly with interelectrode distance. Most significantly, in a portion of cell pairs, correlation strength varied with the direction of the arm movement. We evaluated to what extent correlated activity provided additional information about movement direction beyond that available in single neuron firing rate. A multivariate statistical model successfully classified direction from single trials of neural data. However, classification was consistently better when correlations were incorporated into the model as compared to one in which neurons were treated as independent encoders. Information-theoretic analysis demonstrated that interactions caused by correlated activity carry additional information about movement direction beyond that based on the firing rates of independently acting neurons. These results also show that cortical representations incorporating higher order features of population activity would be richer than codes based solely on firing rate, if such information can exploited by the nervous system.

Although neuronal synchronization has been shown to exist in primary motor cortex (MI), very little is known about its possible contribution to coding of movement. By using cross-correlation techniques from multi-neuron recordings in MI, we observed that activity of neurons commonly synchronized around the time of movement initiation. For some cell pairs, synchrony varied with direction in a manner not readily predicted by the firing of either neuron. Information theoretic analysis demonstrated quantitatively that synchrony provides information about movement direction beyond that expected by simple rate changes. Thus, MI neurons are not simply independent encoders of movement parameters but rather engage in mutual interactions that could potentially provide an additional coding dimension in cortex.It is well known that the discharge of single primary motor cortex (MI) neurons covaries with a number of arm movement-related parameters, including force, position, distance, speed, and direction (1-6). Particular emphasis has been given to the hypothesis that the intended direction of an arm movement is encoded by populations of MI neurons (7). In either single cell or population-coding hypotheses, however, it has been generally assumed that average firing rate mediates the representation of movement parameters. Synchronization of firing of ensembles of neurons has recently received attention as a novel coding dimension because of its prevalence in cortex and its possible functional relevance for perceptual binding (8, 9). Early work evaluated synchronous discharge of pairs of MI neurons largely in the context of identifying local connectivity patterns (10-12), rather than investigating its possible information content. More recent studies by using multi-electrode recordings in frontal cortex have suggested that the synchronous firing of two cells can provide general information regarding the decision to move (13) or expectation of predictable cues (14). However, none of these studies have addressed the relationship between synchrony and movement features. By using a multielectrode array to record from groups of neurons, we provide evidence that neuronal synchronization within MI carries information about movement direction beyond that available from the average firing rate of the same set of neurons. METHODSBehavioral Task. Two macaque monkeys (Macaca fascicularis) were operantly conditioned to perform a reaching task with their left arm. Animals moved a two-joint manipulandum in the horizontal plane to direct a cursor from a central hold position to one of two (left or right) or eight possible (radially positioned) targets that were displayed on a computer monitor in front of the monkey. A trial was composed of three epochs: a ''hold'' period during which time the monkey had to maintain the cursor at the hold position for 0.5 s, a random 1-1.5 s ''instructed delay'' period during which one of the radial targets appeared but movement was withheld, and a movement period initiated by target blinking (time to ...